Re: [PATCH 0/3] block: don't drain file system I/O on del_gendisk

[Christoph Hellwig <hch@lst.de>]

On Mon, Jan 17, 2022 at 05:08:23PM +0800, Ming Lei wrote:
> > We need it to have proper life times in the block layer.  Everything only
> > needed for file system I/O and not blk-mq specific should slowly move
> > from the request_queue to the gendisk and I have patches going in
> > that direction.  In the end only the SCSI discovery code and the case
> > of /dev/sg without SCSI ULP will ever do passthrough I/O purely on the
> > gendisk.
> > 
> > So I think this series is moving in the wrong direction.  If you care
> > about no doing two freeze cycles the right thing to do is to record
> 
> I just think that the extra draining point in del_gendisk() isn't useful,
> can you share any use case with this change?

SCSI disk detach for example is a place where we need it.

> > if we ever did non-disk based passthrough I/O on a requeue_queue and
> > if not simplify the request_queue cleanup.  Doing this is on my TODO
> > list but I haven't look into the details yet.
> > 
> > > 1) queue freezing can't drain FS I/O for bio based driver
> > 
> > This is something I've started looking into it.
> 
> But that is one big problem, not sure you can solve it in short time,
> also not sure if it is useful, cause FS already guaranteed that every
> IO is drained before releasing disk, or IOs in the submission task are
> drained when exiting the task.

Think of a hot unplug.  The device gets a removal even, but the file
system still lives on.

> Firstly, FS layer has already guaranteed that every FS IO is done before
> releasing disk, so no need to take so much effort and make code more
> fragile to add one extra FS IO draining point in del_gendisk().

In the hot removal case the file system is still alive when del_gendisk
is called.

> Also the above two things aren't trivial enough to solve in short time, so
> can we delay the FS draining in del_gendisk() until the two are done?

We already have the draining.  What are you trying to fix by removing it?